If ``getViewer`` is of type ``ITextViewerExtension2`` (which is basically always true since we only support Eclipse 3.6+), you can call ``getViewer.invalidateTextPresentation(offset, length)`` where ``offset`` and ``length`` is the region that needs to be recomputed. You can compute the ``damaged`` region with the upcoming [PositionsChange](https://github.com/scala-ide/scala-ide/pull/309/files#L19R29) (or maybe you could refactor the code to better suit your needs ;-))